Thin film transistor substrates used as display device substrates for liquid crystal display devices and the like are provided with thin film transistors (also referred to as “TFTs”), for example, as switching elements for respective pixels, which are the smallest image unit.
Also, in general, thin film transistor substrates include gate electrodes, source electrodes, and drain electrodes that constitute the above-mentioned TFTs, and wiring lines connected to these electrodes, and as a material to form these electrodes and wiring lines, copper, which has a low resistance and low cost, is used.
Here, insulating layers such as an interlayer insulating layer and a gate insulating layer are formed on an insulating substrate such as a glass substrate, and it is known that when forming a copper film included in gate electrodes and the like on the insulating layers, when performing annealing thereafter, the copper from the copper film is dispersed to the insulating layers, which reduces the insulating properties of the insulating layers.
In conventional configurations, in order to prevent such a dispersion of copper, a barrier metal (dispersion prevention layer) is provided between the copper film and the insulating layer so as to prevent the dispersion of copper. For the barrier metal layer, a metal layer that prevents the dispersion of copper and improves adhesion with the insulating layer therebelow, for example, is used, and this metal layer is made of a metal such as tantalum or nickel, for example.
When forming electrodes and wiring lines made of copper, first, the barrier metal layer is formed on the insulating layer provided on the insulating substrate, and on the surface of the barrier metal layer, a copper film is formed by sputtering. Then, by performing resist patterning and wet etching on this copper film by photolithography using a photomask having a prescribed pattern, a copper seed layer is formed on the barrier metal. Next, on the copper seed layer, a thin copper film is formed by electroless plating, and thus, electrodes and wiring lines are formed of copper (refer to Patent Document 1, for example).